The invention is directed to a twist drill bit that is rotated by a hand-held tool device, the drill bit is designed principally for abrasive cutting or chipping of rock and other rock-like materials, such as concrete and masonry.
Rock drills of this type are characterized by a tool shaft rotatable about an axis and having a shaft core, extending between a tool head that is generally comprised of cutting edges fabricated of hard-metal and a chuck insert end and designed for insertion into the chuck of a hand-held drilling tool, around which several helical discharge grooves are formed. With the rotation of the twist drill around its rotational axis, the outer surface of the shaft, formed by the radial outer surface of the helical lands, describes an essentially cylindrical envelope having the diameter of the bore hole. The helical grooves formed on the drill bit shaft, and the envelope surface of the bore hole, serve to remove the abrasively excavated material.
The area of the shaft situated immediately behind the tool head is subjected to a high rate of wear as a result of the extended period of contact with the rigid walls of the bore hole, whereby the diameter of the envelope surface in such region is reduced and consequently assumes a conical shape. In conjunction with the radial wear on the cutting surfaces on the tool head, and in the case of worn twist drill bits, this results in a seizing of the conical shank at the conical sides of the bore hole when carrying out deeper drilling operations. Furthermore, twist drills having a cross-section of less than 15 mm are prone to blockage as a consequence of the concentration of the cut material in the helical discharge grooves.
DE 19707608A1 discloses a twist drill bit for producing conical holes in non-metallic material, the drill bit being particularly suited for screw connections. To this end the twist drill bit is characterized by a shank tapering conically to a point having spiral-shaped discharge grooves. Twist drills of this type are not suitable for drilling hard material such as rock.
U.S. Pat. No. 5,482,124 discloses a rock drill that is characterized by various increments of the discharge helical grooves along its axis of rotation, whereby the grooves increase in size in the direction of the tool head.
According to CH476559, a rock drill is characterized as having a variable drill shaft diameter along the axis of rotation. In order to achieve a lighter weight and the enhanced drill performance associated therewith, the drill shaft is tapered in the axial mid-section of the shaft, whereas it is reinforced at the transition to the chuck insert end and at the tool head, so that vibration node points that lead to material fatigue can be shifted or better absorbed without damage.
A primary object of the invention is that of reducing the twisting wear in the case of worn twist drill bits. A further aspect is based on the reduction of blockage in the helical discharge grooves.
Generally, in a rock drill bit the width of the outer surface of the helical section expands along the axis of rotation in the direction of the tool head.
Due to the expanded outer surface of the helical section, radial wear is counteracted in the region adjacent to the tool head, whereby an extended life of the twist drill results relative to a certain permissible reduction of the hollow spatial area of the drill bit.
In order to avoid prolonged alternating stress fractures at the vibration nodes, the drill shaft is designed to be advantageously reinforced at the chuck insert end. Of further advantage is the fact that the drill shaft varies along the axis of rotation, whereby it is designed thicker in the regions of the vibrational nodes and, consequently, relative to a certain permissible prolonged alternating stress averaged over its length, is designed more elongated and thus lighter.
Also advantageous is the fact that the reduced cross-section area of the discharging helical grooves achieved by the wider outer surface of the helical section is compensated by the tapering drill shaft diameter towards the tool head.
The cross-sectional area of the discharging helical grooves becomes advantageously larger in the direction of the chuck insert end by virtue of the narrower outer surface of the helical section, whereby the risk of blockage is counteracted. A further advantage is represented by the fact that the pitch of the helical lands along the shaft varies and it decreases particularly towards the chuck insert end, whereby in the case of vertical boring downwardly adequate conveyance of the cut and/or chipped material is achieved, while at the same time it loses kinetic energy along the shaft.